U.S. Pat. No. 4,497,765, hereby incorporated by reference, discloses an apparatus for compression molding plastic closures which has proven to be highly commercially successful. The method of operating the molding apparatus entails depositing a predetermined quantity of molten plastic material, typically polypropylene, in a female mold cavity, with a male mold plunger thereafter inserted into the cavity to compression mold the molten plastic. After cooling, the closure shell thus formed is removed from the mold assembly by opening of the male and female components. In accordance with the above-referenced patent, a sealing liner is similarly compression molded within the molded closure shell, by depositing a predetermined quantity of molten plastic in the shell, followed by compression molding of the liner material with a forming plunger inserted into the shell.
The above-described compression molding of the closure shell (as well as molding of the closure liner) is effected through the use of a rotary turret molding apparatus. The apparatus for molding the closure includes a plurality of cooperating sets of male and female molds, which are positioned about the periphery of a rotating turret or carousel. Relative movement of the male and female mold portions is effected by movement of each mold tooling assembly with respect to cams of the molding apparatus. These cams effect closing of the mold, and compression of the molten plastic material, as well as opening of the molds for removal of the molded closure shells.
Commercial embodiments of this type of molding apparatus are capable of relatively high-speed operation, molding hundreds of closures per minute. To achieve these high rates of production, high speed cutting devices are employed for delivering the predetermined quantities ("pellets" or "charges") of molten plastic material from an associated extruder respectively to each of the female mold cavities. However, attendant to high-speed operation, misplacement of the metered plastic material can result in one of the female cavities improperly receiving a "double pellet", that is, significantly more than the predetermined quantity of molten plastic material. Under these circumstances, cam-actuated closing of the mold assembly can result in excessively high stresses being created within the mold tooling, as well as on the cams of the molding apparatus.
In current machines, this production problem has been addressed by employing pneumatic cylinders to position the compression cam of the molding apparatus. The pneumatic cylinder is pressurized to, in effect, limit the force between the reactive surfaces of the compression cam and the cam follower of the mold assembly. In conjunction with the pneumatic cylinder, a pair of mechanically-actuated limit switches are employed for detecting movement of the compression cam in excess of a normal range, which movement is indicative of a mold having received a significant excess of molten plastic material. The limit switches are sequentially arranged such that actuation of the first switch stops the flow of molten plastic material, while actuation of the second switch, indicating relatively greater movement of the compression cam, stops the apparatus. In conjunction with actuation of one or both of the switches, pneumatic pressure to the pneumatic cylinder holding the cam in position is released, thus acting to prevent excessive strain of the various components.
In practice, proper adjustment of the mechanical limit switches has proven difficult. Not only are the switches subject to vibration which can interfere with their proper positioning, misadjustment of the switches so that they are actuated too easily can result in unnecessary "nuisance trips", while misadjustment such that the switches do not properly react to excessive plastic in the mold can defeat the intended function of the switches.